1. Field of the Invention
The present invention relates to a quadrature signal detector or demodulator which enables extended range operation using analog-to-digital converters having a relatively small number of bits.
2. Description of the Prior Art
Linear detection of intermediate frequency (IF) signals for coherent radar, sonar and communication systems is conventionally achieved using analog inphase and quadrature (I & Q) detectors or demodulators, which convert the IF signal to baseband signals on which phase detection is performed, or with more recently developed IF direct-digital-sampling (DDS) quadrature detectors which sample the IF signal directly at 90.degree. time intervals using high speed analog-to-digital converters. Analog I & Q and DDS detectors are described, for example, in an article entitled "A New Quadrature Sampling and Processing Approach", by H. Liu et al, in IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS, vol. 25, no. 5, Sept. 1989, pp. 733-748.
FIG. 1 illustrates a conventional analog I & Q detector 10 including an input hybrid or signal splitter 12. The splitter 12 receives, for example, an analog IF bandpass signal IF IN from a coherent radar receiver (not shown), and splits the signal IF IN into two identical components which are fed to inphase and quadrature mixers 14 and 16. A 90.degree. hybrid 18 receives a coherent reference signal COHO at the same frequency as the signal IF IN, and splits it into an inphase or SIN reference component which is applied to the mixer 14, and a quadrature or COS reference component which is applied to the mixer 16. The mixers 14 and 16 are typically double-balanced diode mixers, and produce at their outputs phase detected or demodulated inphase and quadrature analog baseband video signals respectively.
The analog inphase baseband signal is fed from the mixer 14 through a low pass filter 20 and amplifier 22 to an analog-to-digital converter 24 which samples the analog inphase baseband signal at periodic intervals and produces an inphase digital baseband video signal I OUT. The analog quadrature baseband signal is fed from the mixer 16 through a low pass filter 26 and amplifier 28 to an analog-to-digital converter 30 which samples the analog quadrature baseband signal at periodic intervals and produces a quadrature digital baseband video signal Q OUT. The amplitude information of the signal IF IN is represented by (I.sup.2 +Q.sup.2).sup.1/2, and the phase information of the signal IF IN is represented by tan.sup.-1 (Q/I), where I and Q are the magnitudes of the signals I OUT and Q OUT respectively.
Analog I & Q detectors have inherent phase errors caused by imperfect quadrature matching of the hybrid components, and by DC offset errors present in even the best double-balanced diode mixer phase detectors. Video amplifier gain mismatches also contribute to I & Q amplitude imbalances which further degrade the resultant I & Q video signals from ideal quadrature.
The quadrature errors associated with analog I & Q detectors can be minimized by optimization of the detection circuitry components. Quadrature errors associated with the hybrids are minimized by phase "trimming" and careful selection of the components. DC offset errors are compensated for by external sensing and feedback circuitry. Variable gain control of video amplifier gain is required to minimize I & Q gain mismatches.
Deviations from ideal quadrature in the I & Q channels (I and Q imbalances) caused by phase and amplitude errors result in the generation of spectral images, which limit the performance of analog I & Q detectors. For minimum achievable phase and amplitude errors of 0.1.degree. and 0.1% respectively, the spectral images of analog I % Q detectors are on the order of -60 dB. Thus, the usable dynamic range of analog I & Q detectors is limited to approximately 60 dB.
DDS detectors do not have the problems associated with analog I & Q detectors. DDS detectors obtain quadrature data by direct digital sampling of the IF signal at 90.degree. time intervals using a high speed analog-to-digital converter, thus eliminating the IF to baseband conversion/detection functions, and their inherent phase and amplitude errors.
A typical DDS detector 32 is illustrated in FIG. 2 in an exemplary application as a detector in a coherent radar system (not shown). A 2ND IF signal at 60 MHz is applied through an amplifier 34 to a mixer 36. A reference signal REF at 10 MHz is divided by a frequency divider 38 having a division ratio of two to produce a 5 MHz signal which is applied to a multiplier 40 having a multiplication ratio of eleven. The resulting signal at 55 MHz is applied through a bandpass filter 42 to the mixer 36.
Mixing of the signals 2ND IF and REF (converted to 55 MHz) produces a 3RD IF signal at 5 MHz which is applied through a low pass filter 44 and amplifier 46 to an analog-to-digital converter 48. The converter 48 operates at 20 MHz (four times the frequency of the 3RD IF signal) to directly sample the 3RD IF signal and produce digital inphase and quadrature signals which are processed by a recursive or non-recursive Hilbert or other applicable filter 50 to produce the digital inphase and quadrature baseband signals I OUT and Q OUT.
The required sampling rate of DDS detectors is four times the IF frequency. Therefore, down conversion to a lower IF frequency is typically required to sample the IF signal directly using state of the art analog-to-digital converters. Because of the speed limitations of current analog-to-digital converters, DDS detectors have limited applications to systems with relatively narrow IF band-widths. For these systems, the DDS performance is limited by the resolution of the analog-to-digital converter and associated aperture uncertainty. The peak signal to RMS noise (dynamic range) of an analog-to-digital converter is determined by the number of bits N, and is approximately equal to 6(N-1). The maximum dynamic range of a DDS detector having a 12-bit analog-to-digital converter, for example, is 66 dB.
DDS detector performance is presently limited by analog-to-digital converter technology. Currently, the state of the art is 12-bit, 20 MHz analog-to-digital converters. Slower, 14-bit analog-to-digital converters which operate at 100 KHz are also available (to obtain higher dynamic range), but these converters cannot be used in DDS detectors for radar systems, which typically have IF bandwidths on the order of MHz. Thus, DDS detector dynamic range is presently limited to 66 dB.
The dynamic range of modern radar receivers must be commensurate with the range of signal levels encountered in present day scenarios. A typical scenario involves the detection of small target returns in the presence of large clutter or interference, typically on the order of 80 dB higher than the target. In order to process signals over such a wide signal range, receivers must achieve greater than 80 dB dynamic range from RF input to digital (I & Q) output.